1. Field in the Industry
The present invention concerns a case hardening steel which gives carburized machine parts having appropriate carbon contents by suppressing excess carburization. The invention concerns also carburized machine parts produced with this case hardening steel.
2. Prior Art
Recently, for production of machine parts with steel by processing the steel to form, for example, a gear, and carburizing the shaped green, vacuum-carburization has been often used instead of conventional gas-carburization. This is because the vacuum-carburization has the following advantages over the gas-carburization:    1) grain boundary oxidation can be avoided, because no oxidation of the material occurs during the carburization carried out under vacuum, and thus, the strength of the product is secured;    2) high temperature carburization can be easily practiced due to the structure of the device for the carburization, and therefore, rapid carburization is possible; and    3) running cost for the carburization is low since the amount of carburizing gas is small.
On the other hand, surface carbon contents of the carburized products tend to be influenced by the shape of the parts, namely, excess carbon may be introduced into the edge-shaped parts, and as the results, local decrease in the strength may be observed due to increase of the amount of the residual austenite and formation of carbides. For the remedy of this disadvantage there has been proposed to remove the excess carbon by carrying out decarburization after the carburization (Japanese patent disclosures Nos. 2003-171756 and 2004-115893). The decarburization runs, however, risk of losing the advantages of vacuum carburization due to not only increase of the process steps but also decrease of the strength caused by grain boundary oxidation during the decarburization.
There is a problem in the carburized products that strain occurs in the parts at quenching step of the carburization, and the strain may cause destruction during use of the machine parts. For the purpose of preventing this it has been proposed to choose a certain alloy composition which may form ferrite in the non-carburized parts so as to convert the structure to ferrite-martensite binary phase (Japanese patent disclosure No. 09-111408). The technology, however, is not helpful to the goal of increasing the strength of the carburized machine parts.
In regard to a case hardening steel with increased strength there was disclosed a technology of making the depth of oxidation at grain boundaries small by dispersing fine TiC so as to enjoy the resulting increase of strength (Japanese patent disclosure No. 2004-3000550). Also, a case hardening steel with increased resistance to temper softening by choosing alloy composition to increase the strength of the tooth flanks such as pitching resistance and abrasion resistance (Japanese patent disclosure No. 2003-231943). These technologies, however, contain no consideration for countermeasure to excess carburization at the edge-shaped parts.
The inventors have made research to seek a way to solve the problem of excess introduction of carbon at the edge-shaped parts in vacuum carburization. Investigation of the mechanism of introducing carbon in the vacuum carburization revealed the fact that carbon is accumulated by formation of carbides during the carbon-introducing step in which carbon is supplied to the surfaces of the machine parts, and then, the carbides decompose in the diffusion step to release carbon, which is supplied to the matrix by being dissolved therein. The inventors considered that the excess carburization in the edge-shaped parts in the vacuum carburization is caused by denser formation of carbides in the edge-shaped parts than in the plane surfaces, and thus, much more carbides accumulate. If, however, carbon contents at whole the surface of the parts are lowered to avoid precipitation of carbides, the carbon contents at the plane surfaces will be extremely low and thus, the hardness and the strength of the carburized machine parts decrease.
Based on the above knowledge, it was found that the highest carbon content in the surfaces of the carburized parts at which no carbide precipitates is 1.1%. On the other hand, the lowest carbon content at which the surfaces of the carburized machine parts have sufficient hardness and resulting sufficient strength was found to be 0.6%.
The inventors further sought alloy composition which may make it easy to control the carbon content in the surface layer of the machine parts to be carburized. This is based on the idea to make, of the carbon to be introduced by carburization, the portion that coming by way of carbides relatively small, and the rest, the portion that coming by way of direct dissolution relatively high, and to realize this by choosing the alloy composition. As the results of the inventors' research on the effect of alloying components it was found that Si and Ni suppress formation of the carbides during carburization, that Cu behaves like these elements, that Cr enhances formation of the carbides, and that Mn and Mo have little influence.